Background Prion diseases such as Creutzfeldt-Jakob disease (CJD) are invariably fatal neurodegenerative diseases that are caused by the autocatalytic formation of a misfolded protein, PrPSc. The biosynthesis and degradation of PrPSc in neuronal cells involves many specific steps, and various classes of therapeutic drugs are available to modulate nearly all of those steps. Unfortunately, no single drug appears to be able to slow the progression of prion disease when administered alone (monotherapy) after the onset of clinical symptoms (which is the relevant clinical scenario when patients are initially diagnosed). Moreover, drug-resistant prions can emerge from a collection of pre- existing PrPSc quasi-species during monotherapy with PrPSc replication inhibitors. Historically, combination chemotherapy has been used successfully to cure a variety of diseases, including many types of infectious diseases and cancer, in which monotherapy failed. In addition, the use of combination regimens typically suppresses drug-resistance in these scenarios by exerting overwhelming selective pressure on replicating pathogens or tumor cells. Proposed Experiments We will compare combination chemotherapy against monotherapy in treating scrapie-infected mice after the onset of clinical symptoms, using a variety of anti-prion compounds previously shown to have efficacy as prophylactic agents. We will specifically study two types of combination regimens: a mixture of different PrPSc replication inhibitors (mAbs directed against different PrP epitopes) (Aim 1), and a mixture of small molecule compounds that target different steps of PrPSc biosynthesis (Aim 2). For each experimental group, we will measure overall survival, and simultaneously perform longitudinal assays of prion titer, drug resistance, and replication kinetics during treatment. If any experimental treatment is able to eliminate prion infectivity (as judged by the longitudinal assays of prion titer), we will also test for the possibility of durable cure by discontinuing therapy. In summary, we propose to perform the first trial of combination chemotherapy to cure symptomatic prion disease using a bona fide animal model of symptomatic prion disease. Our rigorous experimental design will provide clear quantitative data that can be used to critically evaluate the potential utility of this strategy in an authentic clinical setting, and also begin to identify the essential components of an optimal therapeutic regimen.